运行代码时核心转储/分段错误

Question

When I run this I get a warning regarding returning the local address, which is reasonable since all these functions will return a local address. However, I need the functions to return a matrix since I need modulation in my code. The major issue is that I get a core dumped/ segmentation fault error when running this on my PC.

    #include<stdio.h>
    #include<stdlib.h>
    #include<math.h>

    #define N 10

    float *A[N], *B[N], *C[N];
    int i, j, k;

    float** matrix_create(int n){
      float* M[n];
      for (i = 0; i < n; i++)
        M[i] = (float*)malloc(n * sizeof(float));
      return M;
    }

    float** add(float* M1[], float* M2[], int n){
      float** M3 = matrix_create(n);
      for (i = 0; i < n; i++)
        for (j = 0; j < n; j++)
          M3[i][j] = M1[i][j] + M2[i][j];
      return M3;
    }

    float** sub(float* M1[], float* M2[], int n){
      float** M3 = matrix_create(n);
      for (i = 0; i < n; i++)
        for (j = 0; j < n; j++)
          M3[i][j] = M1[i][j] - M2[i][j];
      return M3;
    }

    void print(float* M[], int n){
      for (i = 0; i <  n; i++){
        for (j = 0; j < n; j++)
          printf("%f\t", M[i][j] );
        printf("\n");
        }
    }

    int main(){

      int count = 0, i,j, k;

      for (i = 0; i < N; i++)
        A[i] = (float*)malloc(N * sizeof(float));
      for (i = 0; i < N; i++)
        B[i] = (float*)malloc(N * sizeof(float));
      for (i = 0; i < N; i++)
        C[i] = (float*)malloc(N * sizeof(float));


       for (i = 0; i <  N; i++)
         for (j = 0; j < N; j++){
            A[i][j] = ++count;
            B[i][j] = count;
            }

    float** D = add(A, B, N);
    print(D, N);


    }

Answer 1

Heyo,

The segmentation fault happens as a result of what the compiler is warning you about! In the matrix_create function, you create a local array of float* . In c, when you return an array, it will only return the address of the first element in the array. When the function returns, the stack shrinks, and this memory is no longer part of your program. When you use the [] operator, you're dereferencing the pointer to the address that matrix_create returns. Since this memory doesn't belong to your program anymore, you get a segmentation fault. In order to solve it, you should also allocate the required memory for the float* on the heap with malloc , such that you can keep using the memory after the function returns:

float** matrix_create(int n){
    float** M = malloc(n * sizeof(float*));
    for (i = 0; i < n; i++)
    M[i] = (float*)malloc(n * sizeof(float));
    return M;
}

Keep in mind that you shouldn't just allocate memory on the heap, you should also free it after you no longer need it!